This invention relates generally to methods of forming thin films in manner to obtain desired stoichiometric composition and more particularly to methods of forming ferroelectric thin films having proper stoichiometric composition and excellent crystalline quality.
Generally, MIS type transistors, e.g. MISFETs, have been employed in nonvolatile ROM semiconductor memories, such as, EPROM (Electrically Programmable Read Only Memory) and EEPROM (Electrically Erasable Programmable Read Only Memory). MISFETs function by utilizing a change in the density of the surface charge on the substrate surface immediately beneath the insulator gate film which is retained for a relatively long period of time. This retention ability is brought about by introducing electric charge from the substrate into traps or trap centers in the insulator gate film of the device. An example of such memory devices is disclosed in U.S. Pat. No. 3,646,527. These nonvolatile memories, however, have certain undesirable properties, such as, a relatively high operational voltage levels for read/write functions, for example, about 20 v, and have comparatively high read/write access times, e.g., several tens microseconds in the case of an EEPROM, particularly when compared with the access times of volatile DRAMs and SRAMs. Also, the number of times of read/write functional switching is smal, limited to approximately 10.sup.5 times, thereby rendering such memories of limited lifetime utility.
Nonvolatile ferroelectric type memory semiconductor structures have been proposed having an electrically polarizable ferroelectric layer. These structures utilize the remanent polarization properties of the ferroelectric layer to represent, for example, a binary state, which state is maintain after power has been removed from the circuit. Since the read/write access time and total switching lifetime capacity for nonvolatile ferroelectric memory semiconductor structures is theoretically the same as the above mentioned nonvolatile ROM semiconductor memories, nonvolatile ferroelectric type memory semiconductor structures have the potential to provide a fairly ideal nonvolatile memory structure. For this reason, more attention has been recently directed toward the development of these memory structures.
Examples of nonvolatile ferroelectric type memory devices are disclosed in U.S. Pat. No. 4,149,302, illustrating a plurality of integrated capacitors comprised of an ferroelectric alloy formed on silicon substrate, and in U.S. Pat. No. 3,832,700, illustrating a nonvolatile ferroelectric memory device utilizing a ferroelectric film in place of the insulator gate of a conventional MIS type transistor.
Ferroelectric layers may be deposited by several different methods such as by sputtering, vapor deposition or the like. A popular method of deposition is rf sputtering, such as, disclosed in U.S. Pat. No. 4,437,139 wherein ferroelectric films are deposited at comparatively lower temperatures employing rf sputtering to form amorphous thin films which are thereafter rendered crystalline upon subsequent annealing.
In reality, however, present day nonvolatile ferroelectric type memories have not been of much practical utility because it is difficult to deposit or form ferroelectric thin films that are sufficiently stable. More importantly, their formation to produce a stoichiometric composition of the deposited ferroelectric alloy, such as by rf sputtering, is difficult to obtain in actual practice. For example, where PZT (PbTiO.sub.3 /PbZrO.sub.3) is employed as a ferroelectric film in a memory circuit and the film is produced by rf sputtering, there results an oxide of three elements rather than a desired stoichiometric composition, i.e., the ratio of Pb, Ti, and Zr in the sputtered film does not match the stoichiometric composition thereof. The common procedure for coping with this problem is to add several percent more of the compound, PbO, to the sintered PZT target. Very often, however, the stoichiometric computational ratio is not maintained or ultimately achieved due to the fact that the proper stoichiometric ratios established at the target material do not physically translate via sputtering to the deposited ferroelectric film. Thus, the formed ferroelectric film is an aberration of the stoichiometric target material.
Further, O.sub.2 is often used as or in the sputtering gas medium or atmosphere. However, depending upon the conditions employed for controlling the sputtering process, it frequently occurs that insufficient amounts of O.sub.2 are incorporated in the deposited film so that deposited film is not stoichiometric.
It is an object of the present invention to provide for a method of depositing ferroelectric type films that have excellent stability and certain excellent stoichiometric composition.